1. Field of Invention
The present invention relates to an electro-optical device, such as a liquid crystal device and an electronic apparatus including such an electro-optical device. The present invention also relates to an electroluminescent (EL) device, a device including an electron-emitting element, and an electrophoretic device, such as an electronic paper sheet, wherein the electron-emitting element-including device includes field emission displays and surface-conduction electron-emitter displays.
2. Description of Related Art
Electro-optical devices, such as liquid crystal devices, each including the following components have been known: a pair of substrates and electro-optical materials, such as liquid crystals, placed therebetween. In such electro-optical devices, an image can be displayed by allowing light to pass through the substrates and electro-optical materials. The display of an image can be achieved as follows: the transmittance of light is varied for each pixel by changing the state of such electro-optical materials, thereby displaying different shades of gray for each pixel in a recognizable manner.
As such electro-optical devices, an active matrix addressing electro-optical device having on one of a pair of the substrates, pixel electrodes arranged in a matrix, scanning and data lines extending between the pixel electrodes, and thin-film transistors (TFTs) functioning as pixel-switching elements is known. In the active matrix addressing electro-optical device, each TFT is placed between each pixel electrode and data line for controlling the conduction therebetween. The TFTs are electrically connected to the corresponding scanning lines and data lines. Thereby, the TFTs can be turned on or off using the scanning lines and when the TFTs are turned on, image signals transmitted from the data lines can be applied to the pixel electrodes, that is, the light transmittance can be varied for each pixel.
In the electro-optical devices described above, the above components are arranged on one of the substrates. In order to arrange the components in a two-dimensional manner, a large area is necessary, and therefore there is a problem in that a pixel aperture ratio is lowered, wherein the pixel aperture ratio is defined as the ratio of the area of regions through which light passes to the entire surface area of the substrate. Thus, in related art manufacturing processes, the following configuration has been employed: the components are arranged in a three-dimensional manner, that is, the components are stacked by using interlayer insulating layers. In particular, the TFTs and the scanning lines functioning as gate electrodes of the TFTs are placed on one of the substrates, the data lines are placed thereabove, and the pixel electrodes and the like are placed thereabove. According to this configuration, the devices can be miniaturized and the pixel aperture ratio can be increased by arranging the components in an appropriate manner.
However, in the related art electro-optical devices, there is a problem that the life of the TFTs is relatively short. This is because when a semiconductor layer or gate insulating layer, which is a component of each TFT, absorbs moisture, water molecules diffuse into the interface between the semiconductor layer and gate insulating layer, whereby positive charges are generated and therefore the threshold voltage Vth is increased in a relatively short period. This phenomenon is apt to occur in p-channel TFTs. The short life of the TFTs naturally affects the electro-optical devices as a whole so that the image quality is deteriorated from a relatively early period, and there is a fear that the devices do not operate.